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DB<200> p bab ... DB<200> x / (.) (?! .* \1 ) /gx 0 'a' 1 'b'
DB<211> x / (.) (?<! \1 .* ) /gx Variable length lookbehind not implemented in regex m/ (.) (?<! \1 .* +) / at (eval 261)[C:/Perl_64/lib/perl5db.pl:646] l ine 2.